Chromium electrodeposition



Patented June 12, 1956 CHROMIUM ELECTRODEPOSITION Henry Brown,Huntington Woods, and Donald R. Millage,

St. Clair Shores, Mich., assignors to The Udylite Research Corporation,Detroit, Mich., a corporation of Michigan No Drawing. Application July17, 1953, Serial No. 368,836

5 Claims. (Cl. 20451) Well known, with the evolution of relatively largequantities of hydrogen at the cathode and of oxygen and ozone at theinsoluble anodes. The undesirable spray and mist of chromic acid resultsfrom the violent bursting of the multitudinous gas bubbles of highsurface energy which are released by the electrolysis. The spray andmist resulting from this bubblebursting is of considerable volumebecause of the high current densities used in the plating, the lowefliciency of chromium deposition and the use of insoluble anodes. Dueto the corrosiveness and high toxicity to the workers of this spray andmist and its Well known deleterious contaminating eifect on otherplating baths such as nickel, copper, cadmium and zinc, it is necessaryin large scale production to employ powerful ventilation to continuouslyremove the same as it is formed. For example, to carry away the mistformed in a 2,000 gallon tank installation requires an exhaust of asmuch as 10,000 cubic feet per minute to enable safe continuousproduction. The use of powerful ventilation is expensive not only fromthe standpoint of the initial installation cost and upkeep maintenance,but also because of the abstraction of the heated air in winter.Moreover, not all of the spray and mist is removed from the area of theplating tank even when powerful ventilation is employed and this isespecially true on wide plating tanks. In the usual installation, acertain amount of chromic acid mist does contaminate the air, especiallywhen stray air currents pass over the tank during electrodeposition.From the standpoint of toxicity, the maximum safe concentration ofchromium trioxide is now considered to be 0.1 milligram per cubic meterof inhaled air. In addition to the disadvantages connected with adequateventilation, there is the further disadvantage of the loss of chromicacid in the exhausted air which may amount to about 30% of the chromicacid which is used in the electrodeposition. The amount of chromic acidthus exhausted, together with that which is carried out on the surfaceof the articles themselves,

actually represents more chromic acid than that which is used up in theformation of the chromium coating.

The problems arising as the result of the formation of chromic acidspray and mist in commercial chromium plating, were recognized almostfrom the very inception of chromium plating and various attempts havebeen made to prevent or to greatly minimize the formation of this sprayand mist. As examples of attempted solutions, it has been suggested toform blankets on the surface of the bath by the use of various oils,floating objects such as plastic pieces, and various wetting agentsespecially non-ionic wetting agents. None of these attempts have beenfound to be eminently satisfactory, and all have left much to bedesired. For example, nonionic wetting agents are rapidly oxidized atthe anode, and floating plastic pieces are too readily displaced whenarticles are placed in and taken out of the bath. Heretofore, noordinary compound which is soluble in a conventional aqueous acidichexavalent chromium bath has been found that will prevent or efiectivelyminimize the formation of spray and mist and yet be stable to theextremely powerful oxidizing conditions existing at the insoluble anodesduring the electrodeposition of chromium from such baths.

It is, therefore, the principal object of the invention to provide astable additive for an acidic hexavalent chromium bath which greatlyminimizes the formation of spray and mist during electrolysis of suchbaths with insoluble or highly polarized anodes.

Another important object of this invention is to provide improved acidichexavalent chromium baths and a method for chromium plating whichsubstantially elimis nates the necessity for the use of expensiveventilating procedures currently employed.

Another object is to provide an additive for an aqueous acidichexavalent chromium bath which is capable of producing chromium depositsof increased brightness which persists with increasing thickness of thedeposits.

It has now been found that these and related objects are accomplishedwhen certain cationic surface-active compounds characterized as 1,1dihydro fluorocarbon amines or their salts are added to aqueous acidichexavalent chromium baths of the type conventionally used in chromiumelectroplating.

Broadly the amines which are suitable for the purposes of this inventioncomprise the primary, secondary, tertiary and quaternary 1,1 dihydrofluorocarbon amines containing a saturated fluorocarbon chain of from 3to 7 carbon atoms, and the salts of the primary, secondary and tertiaryamines. Preferred salts are the chromic acid or dichromate salts.Typical examples of these 1,1 dihydro fluorocarbon amines and theoptimum concentrations for use in otherwise conventional hexavalentchromium baths are given in Table I. It is to be understood that theproportions set forth in Table I represent only the optimumconcentrations, and that benefit is received with somewhat lowerconcentrations and that concentrations up to saturation may be used withgood results. Preferably quantities approaching saturation should beemployed for maximum reduction of spray and maximum brightness of plate.I

TABEE I 1,1 Dihydro fluorocarbon Amines grams/liter (1) CF3(CFz)2OHzNH14-12 (2) OF3 CF2 4CH2NHZ 0. 5-3

(3) CF (CFg) CHgNHg 0. 3-1. 5

H (4) CF (CF;) 4CH7N/ 0. 5-2

Y on; G fi zlr fiz -12 (6) v. 'C'EflQEflziQHzN (l.5 2

CHzCH;

(7). i -GEa(CFz]oQE-2N C a 0. 05-0v 5 CHQC'OOH (8) CFflcFzliCHzN cfia 0.1 05 Br 0H:

CH; (9).. ..i QEKCEQQCHINZCHI -O.1-1.-5

CH: ongs-o.

no) cardamom-1f a o;1-.1

(11), G a'(CEi)z'CHzN-QHs 0. 5-4

1i mm The 1,1 dihydrofluorocarbon amines of Table I can he represented'by-th e general formula:

where n..- 2.6 and R1, R2 and R3 are hydrogen or carbonatoms carryinghydrogen, and the sum of the carbon atoms of'R1-l-R2+R3 being not morethan six, and X 'is an anion. Preferably Xis sulfate, acid sulfate,methiosulfate, Qfiuoride or dichromate.

The surface-active cationic portion of the compounds is the dominantportion in the accomplishment of the invention, and the anionic portionis usually dichromate or chromate once the compound is dissolved in theacidic hexavalent chromium baths. It is to be understood that the basicl,1 dihydro fluorocarbon amine ,(that is, the amine without the dottedline attachments to N of R3 and X may be the form in which ,thematerialis added tothe bath. For example, 71,1 dihydro perfluorohexylamineirnaybe addedas such. However, oncedissolved in theacidic bath, {it formsiasalt with the chromicacid ,or .dichromates of the bath. Thissaltformation with the primary amin is also true, of course, with thesecondary and tertiary amines.

The presence in the chromium plating baths .of halide ions other thanfluoride, are not in g,eneral as desirable as ,dichromate, chromate orsulfate, acidsulfate ,or methiosulfate. Also the presence of organicacids such as formic, acetic, etc. are not generally desirable wheninsoluble 4 lead or lead-alloy anodes are used because of their anodicattack on these anodes.

The additives of this invention can be made from fluorocarbon carboxylicacids made by the process as disclosed in U. S. Patent 2,519,983. Withsuch carboxylic acids, amides or nitriles as starting compounds, thevarious members listed in Table I may be synthesized by conventionalmethods employed in organic chemistry.

In addition to greatly reducing the formation of spray and mist, theincorporation of the 1,1 dihydro fluorocarbon amine compounds of thisinvention, for example, 1,1 dihydro perfluorohexylamine in conventionalhexavalent chromimum plating baths has been found to enable the platingof thicker bright chromium deposits over a bright surface before dullingof the chromium sets in. This advantage has been found'to exist incomparison to a conventional hexavalent chromium plating bath or aconventional hexavalent chromium plating bath whichhas been modified .bythe incorporation of a fluorocarbon sulfonic compound. This can'bereadily seen in .chromium plates of 0.05 to .1 mil thickness. Thethicker chromium plate increases the corrosion protection of nickel, orof nickel, nickel-cobalt, nickel-iron covered ferrous surfaces bydecreasing -the porosity of the chromium plate. For example, steel thatis plated with 11.5 mils of bright nickel and over-laid with 0:08to-0.12 mils of chromium, provides greatly superior corrosion protectionin industrial atmospheres than is aiforded when the over-layer ofchromium has a conventional thickness of only .01-.03 mil.

"With the 1,1 dihydro fluorocarbon amines, no excessive trivalentchromium ions are formed in the cathodic processes.

In contrast to the relatively short chain fluorocarbon amines of thisinvention, ordinary amines such as n-octyl amine or -n-dimethy1 n-toctylamine do not prevent spray or -mist,-and-it is necessary to use n-decylamine or longer chain amines -to;prevent spray and mist from the acidichexavalent chromium plating baths. However, these ordinarysurface-active amines are rapidly oxidized anodically in an amount ofabout .1-1 gram/liter in about 4 hours, thus-destroying theirsurfaceeactive properties.

The examples given below set forth formulations of operative chromiumplating baths useful for decorative or engineering purposes. It will beunderstood that other compounds covered by Formula Aand exemplified bythe compounds of Table I maybe used in these typical formulations [inthe "place of the particular examples given. Additionally, it is to'be'understood that mixtures of the compounds 'of this invention may beemployed as well as the single compounds.

Example I HO -2 50 grams/liter chromic acid '(CrOa) 1:53 grams/ literS04 ion 2zgrams/iliter 1,1dihydroperfiuorohexyl amine (Example 2 ofTable I) Temperature 20 -C.-30 C.

Cathode current density-l'OO- 3OO ;amps./ sq. ft. (approximately 1- 030amps] sq. :dm.)

In the formulation of Example I, catalysts other than the sulfate ionmay be employed, such for example as thefluoriideion'or the fluosilicate'i'on'or mixtures thereof. The proportion of fluoride or fluosilicateion employed is an amount 'which provides an equivalent catalytic effect:to that which :is provided by the l.53 grams/liter of the sulfite ion.

Example 11 200-400 vgrams/liter CrOs 2?4 grams/ liter .SOr anion 1gram/liter 1,1 dihydro .perfiuorohexyl trimethyl amine methiosulfate(Example '9 of Table I Temperature 20 C.50 C.

Cathode current density'l50300 amps./ sq. ft.

What is claimed is:

1. A bath for the electrodeposition of chromium comprising an aqueousacidic hexavalent chromium solution containing a compound having theformula:

R1 C 1*;(0 FahcHalfi -Rz 1: \RI

where n=2 to 6, R1, R2, and R3 are selected from the group consisting ofhydrogen and hydrocarbons, the sum of the carbon atoms of R1+R2+R3 beingnot more than 6, and X is an anion, said compound being present insufficient amount to substantially decrease formation of spray and mist.

2. A bath for the electrodeposition of chromium comprising an aqueousacidic hexavalent chromium solution containing at least about 0.05 gramper liter of a compound having the formula:

R1 C FKC FahCHzIYg-Rg X R;

where n=2 to 6, R1, R2, and R3 are selected from the group consisting ofhydrogen and hydrocarbons, the sum of the carbon atoms of R1+Rz+Ra beingnot more than 6, and X is an anion.

3. In a process of electrodepositing chromium from aqueous acidichexavalent chromium solutions, the improvement which consists in addingto the solution a compound having the formula:

R1 N IM HIIF Ra 5: R; where n=2 to 6, R1, R2, and R3 are selected fromthe group consisting of hydrogen and hydrocarbons, the sum 6 of thecarbon atoms of R1+R2+R3 being not more than 6, and X is an anion, saidcompound being added in sufficient amount to substantially decreaseformation of spray and mist.

4. A method of electrodepositing chromium which comprises the steps ofadding to an aqueous acidic hexavalent chromium solution at least about0.05 gram/liter of a compound having the formula:

R1 OFKCFflnCHflTR-Rz X R,

where n=2 to 6, R1, R2, and R3 are selected from the group consisting ofhydrogen and hydrocarbons, the sum of the carbon atoms of R1+R2+R3 beingnot more than 6, and X is an anion, and electroplating chromium from thesolution.

5. In a bath for the electrodeposition of chromium comprising an aqueousacidic solution of hexavalent chromium, an additive in an amount of atleast about 0.05 gram/ liter, said additive consisting of a compoundhaving the formula:

R1 CF;(GF:)CH2ITI\R2 it R;

where n=2 to 6, R1, R2, and R3 are selected from the group consisting ofhydrogen and hydrocarbons, the sum of the carbon atoms of R1+Rz+R3 beingnot more than 6, and X is an anion.

Simons Aug. 22 ,l950 Chester Oct. 13, 1953

1. A BATH FOR THE ELECTRODEPOSITION OF CHROMIUM COMPRISING AN AQUEOUSACIDIC HEXAVALENT CHROMIUM SOLUTION CONTAINING A COMPOUND HAVING THEFORMULA: